Tetrahydrobiopterin (THB), the electron donor for aromatic amino acid hydroxylases plays a pivotal role in regulating the synthesis of biogenic amines. The current work addresses the question of regulation of THB in tissue, whether the level within a cell controls the rate of neurotransmitter synthesis and finally whether administration of this compound to subjects with certain neurotransmitter deficits can affect neurological or behavioral activity. In cultured pineal gland, it was found that Methotrexate did not influence the THB synthesis. Together with other studies on intermediates in the BH4 biosynthetic pathway supportive evidence has been provided for the concept that the pterin ring structure is fully reduced through most of its biosynthetic steps. Other studies with PC-12 cells showed the THB levels were not coordinately regulated with tyrosine hydroxylase and that under some conditions tyrosine hydroxylase subunits could exceed the molecules of BH4. Evaluation of therapeutic use of THB in Parkinson's disease, dystonia, and endogenous depression continued. Blinded evaluation of films of several patients with dystonia confirmed a major neurological improvement in individuals receiving up to 1gm of THB. A second double blind study of 5 patients with endogenous depression failed to reveal significant clinical improvement. Although biochemical analysis showed that insufficient THB had penetrated the CNS.